Potassium ion pre-intercalated MnO2 for aqueous multivalent ion batteries

Manganese dioxide (MnO2), as a cathode material for multivalent ion (such as Mg2+ and Al3+) storage, is investigated due to its high initial capacity. However, during multivalent ion insertion/extraction, the crystal structure of MnO2 partially collapses, leading to fast capacity decay in few charge/discharge cycles. Here, through pre-intercalating potassium-ion (K+) into δ-MnO2, we synthesize a potassium ion pre-intercalated MnO2, K0.21MnO2·0.31H2O (KMO), as a reliable cathode material for multivalent ion batteries. The as-prepared KMO exhibits a high reversible capacity of 185 mAh/g at 1 A/g, with considerable rate performance and improved cycling stability in 1 mol/L MgSO4 electrolyte. In addition, we observe that aluminum-ion (Al3+) can also insert into a KMO cathode. This work provides a valid method for modification of manganese-based oxides for aqueous multivalent ion batteries. Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s12200-023-00093-0.


Preparation of KMO
Potassium ion pre-intercalated MnO2 (KMO) was synthesized by a sol-gel process based on the previous report with a minor modification 1 .Typically, 1.5 g KMnO4 was dissolved in 25 mL deionized (DI) water to obtain solution A. 2.5 g D(+)-Glucose was dissolved into 10 mL DI water to obtain solution B. Then solution A was quickly added into solution B with vigorous stirring for 15 seconds.The mixed solution was allowed to stand until a reddish sol was formed and readily turned into a brown gel in 30 seconds.
The brown gel was allowed to cool down for 30 minutes before it was moved to an oven and dried at 110 ℃ for 12 hours.The obtained dark brown product was then ground and washed with DI water and ethanol for several times, respectively.After being dried in vacuum at 60 ℃ overnight, the final solid KMO product was obtained.

Preparation of VO2
The VO2 was synthesized by a facile hydrothermal method.Typically, the V2O5 powder (0.3638 g) and H2C2O4 powder (0.5402 g) were dissolved into deionized water (8 mL) by constant magnetic stirring for about 1 h at 75 ℃ until the solution turns dark blue.
Then 20 mL 3 % H2O2 was added to the above solution and kept stirring for about 20 minutes.After that, the mixture was transferred into a 50 mL autoclave with a Teflon liner and kept at 180 ℃ for 10 h.Finally, the precipitate was collected by centrifuging and washing with deionized water and ethanol several times, and then dried at 60 ℃ for 12 h under a vacuum.

Electrochemical characterization
All the electrochemical tests were performed in a three-electrode cell.To prepare KMO working electrode, 70 wt.%KMO, 20 wt.% acetylene black and 10 wt.% carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) (the mass ratio of CMC/SBR is 2:1) binder were mixed and dispersed into deionized water to form a uniform slurry.Then the slurry was coated onto 10 mm diameter carbon paper, followed by drying in vacuum at 80 ℃ for 12h.The mass loading of KMO working electrode is about 1.5 mg cm -2 .The VO2 electrode was prepared through the same method.The counter electrode is activated carbon (AC) films, prepared by mixing AC, acetylene black and polytetrafluoethylene (PTFE) in a mass ratio of 8:1:1.Ag/AgCl electrode is chosen as the reference electrode.As for ex situ XRD test, KMO free-standing film electrode was also prepared as the working electrodes, which employed PTFE as the binder.

Fig. S1 TG curve of KMO.
As the molar mass of K0.23MnO2 is certain, the content of crystal water can be calculated from the equation: , where m and M present mass and molar mass, n present the content of crystal water.It should be noted that the errors of Mg may be due to the large dilution multiples because of its high concentration in electrolyte.However, due to the same dilution condition of Mg in discharge and charge states, the change trend of Mg is still significant.

Table S1
Element contents by EDS mapping.Where m and M denote mass and molar mass of each element, respectively.TableS3Element contents of electrolyte by ex situ ICP-OES.